In my U.S. Pat. No. 5,163,728 I describe a mining apparatus for working a horizontally and longitudinally extending seam having a vertical face. This machine has a longitudinal row of walking props extending along the face and each having a roof-engaging cap, a floor-engaging foot, and a jack vertically interconnecting the respective cap and foot and expansible to press the cap up and foot down. Upper and lower long-wall mining machines corresponding generally to those described in commonly owned U.S. Pat. No. 4,883,322 of G. Blumenthal each have a predetermined height substantially less than the distance between the mine roof and floor and are each provided with a cutter for working the face and a longitudinal conveyor for carrying off material cut from the face. The upper machine is suspended from the caps well above the floor level and the lower machine is carried on the feet of the props below the caps thereof generally at floor level and spaced back toward the props from the face so that the upper machine moves on a step above the lower machine and the two machines can simultaneously work the seam at respective vertically and horizontally offset levels.
With this system, therefore, conventional-height mining machines can work together on a face that is much higher than either of them could work alone. The machines can make respective cuts that vertically overlap or these cuts can be vertically spaced. In the latter case the intervening ridge of rock can easily be broken down to be taken up by the lower machine, and in practice normally will fall down as it is undercut.
The procedure of my prior invention is different from the underhand stoping normally done in open-pit operations. In it the individual levels are worked independently. According to my earlier invention they are cut away and advanced simultaneously and synchronously. This preserves the efficiency of long-wall-mining with a single double-height cutter while avoiding the extra expense of this equipment.
The main problem with this prior-art system is that it is very complicated, with two complete cutter/conveyors. Each of these two relatively complex pieces of equipment must be driven and maneuvered, making the mining apparatus bulky, service-prone, and expensive.